f97.beERIK SCHMIDT’S WORKSHOP
← All terms

Glossary · Energy industry & energy transition

Heat Simultaneity Factor

Heat simultaneity degree Coincidence factor heat demand heat pump heat network Heating load grid planning

The heat simultaneity factor is the ratio between the heat output that a group of consumers actually draws at the same time and the sum of their individually assumed nominal or peak outputs. This usually refers to the point in time at which the combined load of the group is highest. In the technical literature the quantity is usually called the simultaneity factor (abbreviation GLF or g). It has no unit and is stated as a factor or as a percentage. A value of 0.7 means: for the combined peak, 70 percent of the summed individual peaks is assumed. For a single consumer the factor is 1 by definition.

Definition and calculation method

Formally, the numerator is the sum of the outputs of all consumers at the time of the combined maximum load. The denominator is the sum of the nominal outputs. The reference point is important: for sizing a heat generator, the group comprises all consumers of the network. For an individual pipe section, only those consumers count whose heat flows through that section. A network therefore does not have just one single simultaneity factor. Each branch has its own, and it is higher the fewer consumers the branch supplies.

The factor describes power, not energy quantity. For heat, this is the thermal output in kilowatts. Related terms must be kept apart from it. The heating load is the output that a single building needs under defined design conditions. The heat demand, by contrast, is a quantity of energy over a period of time, for example kilowatt-hours per year. The load profile shows how the load is distributed over time. The simultaneity factor asks what part of the individual power requirements occurs at the same time in the group. This distinction is important: pipes, pumps, transfer stations and generators are sized by power, but billing is usually based on kilowatt-hours.

The denominator is not as clear-cut as it looks. Nominal outputs can come from standard heating load calculations, from contractually agreed connection capacities or from measurement data. Contractually agreed values are often set too high. Part of the apparent simultaneity effect then merely compensates for overly generous individual values. Conversely, short peaks can exceed the nominal output of a transfer station, for example when charging a hot water storage tank with a low flow temperature on the building side. How the denominator was determined should therefore always be stated as well.

Why the combined peak is lower – and when it is not

The peaks of individual buildings do not occur at exactly the same time. Buildings differ in insulation, thermal storage mass, orientation, use, control and internal heat gains. In addition there are short, behaviour-dependent peaks in hot water demand. In a larger group these differences overlap, and the combined curve becomes smoother. Heat, however, depends strongly on the weather. In frost, the demand of all buildings in the same area rises at the same time. The smaller and more uniform a group is and the more similarly the buildings are controlled, the closer the factor is to 1. Adopting a value from a different neighbourhood, building stock or climate can misrepresent the load considerably.

Empirical orders of magnitude in local heating networks

A frequently cited study by Winter, Haslauer and Obernberger from 2001 evaluated hourly values from 559 transfer stations in two Austrian biomass local heating networks over about one and a half years. Result: the factor falls with the number of consumers. For small groups of fewer than about 15 consumers it falls only slowly. From about 200 consumers it settles at around 0.47. The authors derived from this an approximation formula that depends only on the number of consumers. According to them, it is valid for 1 to 200 consumers. For the Tamsweg network as a whole, a factor of about 0.51 resulted from the generator data.

These figures are empirical values and not standard values. They come from specific networks, building stocks and winters around 1998/99, and are based on hourly averages and on filtered peaks (quantiles). For groups of fewer than 75 consumers they apply only if the nominal outputs differ by no more than a factor of 2. They cannot be adopted unchecked for today's new development areas with very low heating loads, where hot water accounts for a larger share, or for networks with different control.

An example from the same study shows the biggest pitfall: individual large consumers. Assume a branch supplies 50 houses of 10 kW each and one large customer of 1 MW. For 51 consumers the formula gives about 0.73, i.e. roughly 1,095 kW. However, if the large customer alone is already running at full load, the 50 houses additionally need about 360 kW. The branch would thus be undersized. Large individual consumers should therefore be considered separately and must not be absorbed into the group's flat-rate factor.

Relevance for heat networks

For a heat network, the simultaneity factor is a central design parameter. It determines how large generators, pipes, pumps and storage tanks become. If full simultaneity is assumed, pipes and plants usually become larger than necessary. This increases investment costs and, with larger pipes, also heat losses. If the factor is set too low, there is a risk of undersupply on cold days, flow temperatures that are too low at the end of the network, and loss of comfort. The task is therefore not to choose the lowest possible factor. It must be justified on the basis of building types, use, design temperature, hot water share, large consumers and control, ideally with measurement data from comparable networks.

Heat pumps: from thermal to electrical simultaneity

With the heat pump, the simultaneity factor also takes effect in the electricity grid. What matters to grid operators is not the annual heat consumption but the electrical power occurring at the same time, for example at the local distribution transformer or in a line section. The electrical load does not simply follow the thermal load. In cold weather the heating load rises, and at the same time the efficiency of many air-to-water heat pumps falls. In addition, electric heating elements may switch on. The electrical peak can therefore rise more than the heat side would suggest. Ground-source heat pumps usually behave more benignly in frost. Buildings with low system temperatures need less electricity for the same heat. A thermal factor alone is therefore not sufficient for electricity grid planning. It must be combined with assumptions on efficiency, auxiliary heating, storage and control.

Control can shift loads, but can also create new simultaneity. Storage and the thermal inertia of buildings make it possible to shift power for a short time without it becoming noticeably cooler. If, on the other hand, many systems start in response to the same signal, for example after a blocking period, a curtailment or a price signal, a new, synchronous peak can arise. Whether flexibility relieves the grid thus depends on how it is coordinated, for instance through grid-serving control with time-staggered release.

Legal reference in Germany: § 14a EnWG

In Germany the term simultaneity factor also appears in regulation. There, however, it is narrowly delimited and is not the same as the planning factor for heat networks. Under the rules of the Bundesnetzagentur on § 14a EnWG (determination BK6-22-300), the following applies to controllable consumption devices that are put into operation from 1 January 2024, are connected to the low-voltage grid and have a grid connection capacity of more than 4.2 kW, including heat pumps: the grid operator may temporarily curtail their electricity draw if overloading of the grid is imminent. A minimum power of 4.2 kW must always remain available, however.

If several such devices are controlled together via an energy management system, the grid operator calculates the minimum power for all of them together. In doing so it applies a simultaneity factor that is graduated by the number of devices. According to a VDE FNN guidance note, the Bundesnetzagentur initially set 0.8 for two devices. With each additional device the factor falls by 0.05, until it remains constant at 0.45. With direct control, by contrast, 4.2 kW per device applies as a rule. Special rules apply to large heat pumps. These values apply only to the minimum power during curtailment and must not be adopted as a simultaneity factor for grid planning.

Common misunderstandings

  • Not a constant: The factor does not apply as a fixed value to an energy carrier. It depends on the group, the time resolution, the weather year, the design case and the mode of operation. Daily averages smooth out peaks; hourly or quarter-hourly values make them visible.

  • Not arbitrarily transferable: A value for 200 mixed residential units does not apply to ten identical single-family houses. Groups with consumers of very different sizes require separate consideration.

  • Thermal is not electrical: The same heat output does not mean the same electricity consumption. Heat source, flow temperature and auxiliary heating change the electrical simultaneity.

  • Planning factor is not regulatory factor: The graduated factor from the § 14a determination governs minimum power during curtailment. It does not describe a measured simultaneity of heating loads.

Interfaces and classification

The term sits at an interface. Building planners calculate heating loads, heat network operators size networks and generators, distribution grid operators assess electrical connection capacities, and municipal heat planning considers entire areas and supply options. If these levels work with different assumptions, contradictions arise. A building can be correctly sized on its own while the sum of many systems in a street overloads the grid. Conversely, a flat-rate assumption by the grid operator is too coarse if it disregards building characteristics and storage possibilities.

Used correctly, the heat simultaneity factor denotes neither the heat consumption nor the individual heating load. It describes what share of many individual outputs counts simultaneously in the common design case. It becomes meaningful only when the system boundary is disclosed: which buildings, which time resolution, which weather, which technology, which control and which reference power in the denominator. Without this information, a useful planning quantity becomes a seemingly precise number with unclear content.

Sources

Related glossary entries

Edited by eCMS on 12.11.2026 05:05

© 2026 Erik Schmidt. All rights reserved. Quotations with a link to the source are welcome.